setup: init-db e reset-db stampano head in un thread (TieMeasureFlow#80)

_stamp_head esegue env.py di alembic, che fa asyncio.run() e dentro il loop
di FastAPI dava RuntimeError (500). Ora gira con asyncio.to_thread. Nuovo
test_setup_stamp.py senza monkeypatch di _stamp_head, su file SQLite.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/backend/api/routers/setup.py

@@ -3,6 +3,7 @@
 Protected by SETUP_PASSWORD env var. When empty/None, all endpoints return 404.
 NOT protected by API Key auth (standalone access).
 """
+import asyncio
 import hmac
 import random
 from datetime import datetime, timedelta
@@ -312,7 +313,9 @@ async def init_db(body: PasswordBody):
     await _require_not_initialised()
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
-    _stamp_head()
+    # #80: env.py di alembic fa asyncio.run(), vietato dentro il loop di FastAPI;
+    # in un thread non c'e' un loop attivo e funziona.
+    await asyncio.to_thread(_stamp_head)
     return {"status": "ok", "message": "Database tables created successfully"}
 
 
@@ -748,6 +751,8 @@ async def reset_db(body: ResetDbBody):
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.drop_all)
         await conn.run_sync(Base.metadata.create_all)
-    _stamp_head()
+    # #80: env.py di alembic fa asyncio.run(), vietato dentro il loop di FastAPI;
+    # in un thread non c'e' un loop attivo e funziona.
+    await asyncio.to_thread(_stamp_head)
 
     return {"status": "ok", "message": "Database reset successfully"}